Optional Nikon eyepiece diopters allow you to optimize your SLR
viewfinder's sharpness in conjunction with your individual eyesight.
Nikon SLR cameras (without built-in adjustable eyepiece correction) have
a base diopter value of minus one. Minus one is considered the optimum
starting point for the majority of SLR users with normal vision. The
combination of the camera's viewfinder components and a clear (standard)
eyepiece form a virtual image of a subject on the camera's focusing
screen that is the approximate equivalent of a viewing distance of one